San Joaquin Near-River Groundwater Models

California

SSP&A has worked on the San Joaquin River to evaluate hydrologic conditions related to river restoration in a series of projects from 1999 to the present. In 1999-2000, SSP&A was retained by the U.S. Bureau of Reclamation to develop fine resolution groundwater models of the near-river zone, linked to existing HEC surface-water models along a 150-mile river reach, from Friant Dam to the Merced River. This groundwater model was developed to support evaluations of how much surface flow was needed to sustain desired hydrographs for repatriation of salmon.

SSP&A developed a technical approach that involved the use of very fine resolution groundwater models for specific river reaches, embedded within a structure incorporating regional groundwater conditions and with flow-dependent river stage and width identified with existing HEC surface-water models. The groundwater models simulate seasonal fluctuations of the water table in the riparian zone in response to user-specified river flow and regional groundwater conditions. A vadose zone package was developed and applied with MODFLOW as part of this effort. Example model simulations illustrated the sensitivity of groundwater levels and river gain/loss rates to antecedent river flow and regional groundwater conditions.

In 2005, SSP&A was retained on behalf of the Friant Water User’s Association to provide litigation support in evaluating losses from and gains to the San Joaquin River that would be associated with enhanced flows for re-establishment of an anadromous fishery on the San Joaquin River. This work involved evaluation of additional monitoring data, recalibration of models, and evaluation of various proposed restoration hydrographs to assess the feasibility of the delivery or maintenance of surface water flows at various flow levels. Time steps for the analyses ranged from hours to several days, with shorter time steps for ascending and descending hydrograph limbs to capture dynamics associated with significant changes in flow.

In 2008 and 2009, SSP&A applied the groundwater models to evaluate seepage losses associated with planned restoration hydrographs as part of a PEIS under NEPA/CEQA and to provide recommendations for monitoring locations and modeling tools for future evaluations of groundwater-surface water interactions.